In the field of assembling sheet metal parts, and more generally parts of composite material used in car bodyparts, there are presently two main methods of performing such positioning for assembly, in particular by welding.
The first method consists in bringing the parts to be assembled to an assembly station and in holding them relative to one another by means of positioning tools. These tools belong to a station, e.g. along a general assembly line, and their positions are determined in a fixed frame of reference of the station in which the assembly tools also move. Under such circumstances, the machine or assembly line is specific and can accept only one model of an object, e.g. a car bodypart. A substructure of bodyparts is brought to the station by means of a bottom conveyor and is then separated from the conveyor by members for putting the substructure in a reference position in the frame of reference of the station (e.g. abutments, centering fingers, vertical supports, . . . ). Other conveyors bring sides of the body to the station where they are taken over by moving tools for placing the body sides in positions that are determined in a frame of reference of the station, and thus relative to the substructure. Finally, other tools bring the roof, cross-members, and the like in the same way.
Such a single-shape assembly station can build only one shape, with the advantage of exactly reproducing the same shape for each product going through it. This specificity of the machine nevertheless presents the drawback of being incapable of accommodating model changes without major action being taken on the tooling. Also, it is not capable of being adapted to assembling more than one model in a single stream.
Attempts have been made to make such an assembly line more flexible, and they have given rise to a second method which consists in making subassemblies of assembled parts storing them on a pallet (bottom of bodywork, side, front block, . . . ) and then bringing together all of the pallets, required for making the bodywork, to a single station where the subassemblies are moved towards one another into positions determined by apparatus belonging to the positions station. Each pallet is roughly in the form of a frame or a tray having on one face thereof features for positioning and holding the assembled parts, which are permanent and dedicated to the assembled parts, while the other face of the pallet has standard features which are to be found in identical manner on all pallets of the same type (substructure, side of bodywork, etc.). The standard features are designed to co-operate with simple referencing tools belonging to the station. Such a station can thus accept pallets carrying different models since the tools of the station only "see" a single common pallet face regardless of the model. Only computer control relating to paths or trajectories needs to be changed from one model to another, and that can be done very much more quickly than changing the tooling. Positioning is generally performed by locking the pallets to means fixed to the station. Such a technique can be said to be "multi-shape" since even two identical products (the same model of vehicle) are handled by means of distinct pallets which are necessarily different given the dispersions of dimensions between identical parts constituting two pallets. Thus, two same-model products are not necessarily of exactly the same shape, and the resulting manufacture is of poorer quality than is the single-shape type manufacture.
In addition, pallets are extremely heavy and bulky. They require powerful handling tools to be installed that are likewise bulky and that impede access to the station. It is also necessary to have a large number of pallets in order to maintain rotation thereof between upstream assembly lines, where they are fitted with the parts they are to carry, and the general assembly line. As a result this type of installation is expensive and that limits the advantage of using it.
There thus exists an unsatisfied need for an assembly line that assembles bodyparts, that is versatile in use and relatively cheap, while nevertheless conserving the qualities of a single-shape type system.